Method for the production of low-viscous water soluble cellulose ethers

ABSTRACT

A process for preparing low-viscosity water-soluble cellulose ethers by the oxidative decomposition of higher-viscosity cellulose ethers with hydrogen peroxide is described. The process involves: (a) forming, under conditions of intensive mixing and at temperatures of 65-125° C., a mixture of, (i) one or more higher-viscosity cellulose ethers, and (ii) an aqueous solution of hydrogen peroxide, the proportions of the mixture being selected in such a way that the hydrogen peroxide content is 0.1-10 wt. % in relation to the dry cellulose ether, the solids content of the mixture is at least 25 wt. % in relation to the total weight of the mixture; and (b) agitating continuously the mixture of step (a) at temperatures of 65-125° C. until at least approximately 90% of the hydrogen peroxide has been spent.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present patent application claims the right of priority under 35U.S.C. 119 and 35 U.S.C. 365 of International Application No.PCT/EP99/08779, filed 15 Nov. 1999, which was published in German asInternational Patent Publication No. WO 00/32636 on 8 Jun. 2000, whichis entitled to the right of priority of German Patent Application No.198 54 770.6, filed 27 Nov. 1998.

The technological properties of cellulose ethers are highly dependent onthe viscosity of their solutions. Although primarily medium-viscositycellulose ethers, i.e. those with average molecular weight areprocessed, high- and low-viscosity cellulose ethers have neverthelessalso achieved importance.

Low-viscosity cellulose ethers, which also have a low molecular weightin comparison with medium- and high-viscosity cellulose ethers, can inprinciple be manufactured in two different ways. Either a low-molecularalkali cellulose is taken as the basis and etherified, or a finishedcellulose ether is broken down to the desired molecular weight.

Using a low-molecular alkali cellulose as the basis and producing acellulose ether by etherification makes the subsequent cleaning processmore difficult. The cellulose ether contains a considerable number ofshort-chain components, which are swollen greatly or washed out by thewashing media.

The second possible method mentioned, of breaking down higher-molecularcellulose ethers into low-molecular, low-viscosity cellulose ethers canbe achieved by the action of oxidising agents, for example hypochloriteor hydrogen peroxide.

The oxidative decomposition of high-viscosity cellulose ethers can becarried out after the cleaning process. This avoids washing losses anddifficulties during the washing process.

The specifications listed below give a summary of the processescurrently used to break down high-viscosity cellulose ethers afteretherification and washing:

DE 2 016 203 from The Dow Chemical Co. claims a process for reducing theviscosity of cellulose ethers with hydrogen peroxide. For this process asubstantially dry, free-flowing cellulose ether with a water content ofless than 5 wt. % is mixed with a 10 to 50% hydrogen peroxide solutionand the mixture obtained is heated to 50 to 150° C.

DE 1 543 116 from Kalle AG claims a process for the production oflow-viscosity cellulose ethers by oxidative decomposition ofhigher-viscosity cellulose ethers with hydrogen peroxide. This processis characterised in that, a higher-viscosity cellulose ether is mixedwith an aqueous solution of hydrogen peroxide, the water content of themixture not exceeding 75 wt. % in relation to the total quantity. Themixture is then dried at temperatures of 100° C. to 250° C. until thehydrogen peroxide is spent. Here, the loss of moisture and of hydrogenperoxide run virtually in parallel with the reduction in viscosity.

These processes have in common, that a low-viscosity cellulose ether indry powder or granule form results directly from the decompositionreaction. Either drying is carried out before the decomposition reactionand the process is carried out with low moisture contents, or theprocess begins with a wet product and ends with low moisture contents.The loss of viscosity then runs virually in parallel with the loss ofmoisture.

The object was to provide a process which allows the viscosity to be setimmediately after washing of the cellulose ether in such a way that thesubsequent drying, shaping (grinding, granulation) and mixing are notaffected and that the decomposition reaction is not affected by thesubsequent process steps drying, shaping (grinding, granulation) andmixing.

This object was achieved in that a higher-viscosity water-solublecellulose ether as obtained after washing, was mixed with an aqueoussolution of hydrogen peroxide, the dry content of the mixture notexceeding 25 wt. % in relation to the total quantity. The mixture isthen continuously agitated at temperatures of 65-125° C., preferably75-100° C., until the hydrogen peroxide is spent, and then dried.

By this process a low-viscosity water-soluble cellulose ether isobtained. Surprisingly the subsequent process steps for the productionof cellulose ethers ready-for-sale, such as drying, shaping (grinding,granulation) and mixing are not affected by the decomposition reaction.The degree of moisture and grinding can be set independently of thereduction in viscosity.

Low-viscosity cellulose ethers are deemed here to be cellulose ethers ofwhich 2% aqueous solutions have viscosities of 2 to 400, in particular 2to 100 mnPa/s (Haake Rotovisko) at 20° C. and a shear rate of 2.55 s⁻¹.A higher-viscosity ceflulose ether is deemed here to be a celluloseether of which 2% aqueous solutions have a viscosity of 100 to 100,000,preferably 400 to 20,000 mPa/s at 20° C. and at a shear rate of 2.55s⁻¹. Here the viscosity reduction in the end product as compared withthe raw material, brought about by the process according to theinvention, preferably amounts to at least 50%, in particular 70%, andmore particularly 98%.

Ionic or non-ionic cellulose ethers may be used as raw materials, suchas preferably carboxymethyl cellulose, hydrophobically modifiedcarboxymethyl cellulose, hydroxyethyl carboxymethyl cellulose,sulfoethyl cellulose, hydrophobically modified sulfoethyl cellulose,hydroxyethyl sulfoethyl cellulose, hydrophobically modified hydroxyethylsulfoethyl cellulose, hydroxyethyl cellulose, hydrophobically modifiedhydroxyethyl cellulose, methyl cellulose, methyl hydroxyethyl cellulose,methyl hydroxyethyl sulfoethyl cellulose, hydrophobically modifiedmethyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose,hydroxypropyl cellulose and mixtures or derivatives thereof. Preferredraw materials are in particular methyl hydroxyethyl cellulose or methylhydroxypropyl cellulose. It is advantageous to use water-wet filtercakes of these cellulose ethers, as obtained after washing andseparation.

The process can easily be incorporated into the normal method ofproduction of a cellulose ether. The higher-viscosity raw material iscentrifuged after washing to a dry content of 25 to 80 wt. % in relationto the total weight.

Then an aqueous solution of hydrogen peroxide at temperatures of 65 to125° C., is incorporated intensively, optionally step-by-step, theproportions of the mixture being selected in such a way that thehydrogen peroxide content is 0.1 to 10 wt. % in relation to the drymatter and the dry content of the mixture does not fall below 25 wt. %in relation to the total quantity. The mixture is then agitatedcontinuously at temperatures of 65-125° C., preferably temperatures of75-100° C. until approximately 90% at least, preferably 95% at least ofthe hydrogen peroxide used is spent. It is preferred more particularlythat the mixture be agitated continuously until the hydrogen peroxidehas been fully spent. The other process steps such as the addition ofadditives, crosslinking with di-aldehydes, compression, drying andgrinding can then be carried out in the usual way.

A higher-molecular cellulose ether with a dry content of 35 to 80 wt. %,in particular 40 to 55 wt. % in relation to the total quantity, ispreferably used in the process. 0.1 to 10 wt. % hydrogen peroxide inrelation to the dry cellulose ether, preferably 0.2 to 2.5 wt. %, inparticular 0.5 to 1.8 wt. % in relation to the dry cellulose ether isused to break down the higher-viscosity cellulose ethers.

Depending on the raw material, products with 2 wt. % aqueous solutionshaving acid pH values of 3 to 5 result from the decomposition reaction.It has proved useful here, before, during or after the decompositionreaction, but in any case before any further processing step such asdrying or shaping, to set the pH value of the product.

Particularly good results are achived if the pH value is set after thedecomposition reaction. The pH value is set using aqueous solutions ofsalts, which have a pH value of 5 to 12 and optionally contain all orpart of the required hydrogen peroxide. These solutions are added to thereaction mixture in such quantities, that the pH value of the mixtureafter addition is set at more than 4.5, preferably 6 to 7. Aqueoussolutions of sodium dihydrogen phosphate, sodium hydrogen phosphate,sodium phosphate, sodium carbonate and sodium hydrogen carbonate oraqueous solutions of mixtures of these salts are advantageously used.Alkali salts of weak acids such as those of citric acid or succinic acidin the form of aqueous buffer solutions can also be used.

An advantage of the claimed process is that the actual reduction inmolecular weight is completely separated from subsequent drying. Thishas the advantage that any type of drying equipment, with varyingresidence time requirements of the cellulose ether particles to bedried, can be used without affecting the decomposition reaction.Furthermore, only one piece of equipment, the mixer in which thedecomposition reaction is to be carried out, is affected by thecorrosive properties of the hydrogen peroxide incorporated. Inparticular, it is possible to incorporate additives and modifiers afterthe decomposition reaction, but before drying, into the solvent-wet(e.g. water-wet) cellulose ether. Here, the group of dialdehydes (e.g.glyoxal) is mentioned in particular. These compounds are used to producesolvent-inhibited cellulose ethers. They cannot be used together withthe hydrogen peroxide required for the decomposition reaction because oftheir sensitivity to oxidation. It is also possible to mix in oligomericor polymeric oxidation-sensitive substances (e.g. polysaccharides,polysaccharide ethers, polyvinyl alcohol, polyester, polyamide) afterthe decomposition reaction and before drying.

The following examples explain the present invention further.

EXAMPLES 1-4

5 kg quantities of methylhydroxyethyl cellulose with a methoxy groupcontent of 24.2-30.5% and a hydroxyethoxy group content of 7.5-14.8% anda moisture content of 50-53 wt. %, in relation to the total quantity andwith a viscosity as given in the following table, measured on 2 wt. %aqueous solutions at 20° C. and at a shear rate of 2.55 s⁻¹ (HaakeRotovisko), were sprayed with 800 ml aqueous hydrogen peroxide solution.The mixture thus obtained was agitated continuously for 6 hours at 75°C. and then dried.

The quantities of hydrogen peroxide used, the initial viscosity and thefinal viscosity are shown in the table. The quantities stated relate tothe dry methylhydroxyethyl cellulose.

Initial viscosity H₂O₂ added Final viscosity Number (mPa/s) wt. %(mPa/s) 1 7,100 0.5 134 2 7,100 1.0 70 3 7,100 1.5 21 4 400 1.5 6

Examples 5-7

5 kg quantities of methylhydroxyethyl cellulose with a methoxy groupcontent of 21.4-26.1% and a hydroxyethoxy group content of 5.9-9.8% anda moisture content of 52 wt. % in relation to the total quantity andwith a viscosity as given in the following table, measured on a 2 wt. %aqueous solution at 20° C. and a shear rate of 2.55 s⁻¹ (HaakeRotovisko), were sprayed with 500 ml aqueous hydrogen peroxide solution.The quantity of hydrogen peroxide used was 1 wt. % in relation to thedry methylhydroxyethyl cellulose. The mixture thus obtained was agitatedcontinously until the hydrogen peroxide had been spent and then dried.

The reaction temperatures, reaction times, initial and final viscositiesin each case are shown in the table.

Reaction Initial viscosity Final viscosity temperature Reaction Number.(mPa/s) (mPa/s) (° C.) time (h) 5 6,000 40 75 6 6 6,000 34 85 5 7 6,00036 95 3

Examples 8-10

5 kg quantities of methylhydroxyethyl cellulose with a methoxy groupcontent of 21.4-26.1%, a hydroxyethoxy group content of 5.9-9.8% and amoisture content of 52 wt. % in relation to the total quantity and witha viscosity as given in the following table, measured on a 2% aqueoussolution at 20° C. and a shear rate of 2.55 s⁻¹ (Haake Rotovisko), weresprayed with 500 ml aqueous hydrogen peroxide solution. The mixture thusobtained was agitated continuously for 3 hours at 95° C. until thehydrogen peroxide was spent and then sprayed with 250 ml of an aqueoussolution of sodium hydrogen phosphate and sodium carbonate and mixed fora further 60 minutes. It was then dried.

The initial and final viscosities in each case, the quantities of sodiumhydrogen phosphate and sodium carbonate used, the pH values of 2 wt. %solutions of the products and the quantities of hydrogen peroxide usedare shown in the table. The quantities given relate to the dry methylhydroxyethyl cellulose.

Sodium Sodium pH-values Initial Final hydrogen car- of 2 wt. % H₂O₂-Num- viscosity viscosity phosphate bonate aqueous added ber (mPa/s)(mPa/s) (wt.. %) (wt. %) solutions (wt. %) 8 6,000 36 0.25 0.2 5.6 1.0 96,000 29 0.25 0.3 5.9 1.0 10 6,000 19 0.25 0.5 5.0 1.5

Examples 11-12

5 kg quantities of methylhydroxyethyl cellulose with a methoxy groupcontent of 24.2-30.5% and a hydroxyethoxy group content of 7.5-14.8% anda moisture content of 50-53 wt. % in relation to the total quantity andwith a viscosity as given in the following table, measured on 2 wt. %aqueous solutions at 20° C., and at a shear rate of 2.55 s⁻¹ (HaakeRotovisko), were sprayed with 800 ml aqueous hydrogen peroxide solution.The quantity of hydrogen peroxide used was 1.5 wt. % in relation to thedry methylhydroxyethyl cellulose. An additional 0.5 wt. % (in relationto the dry methylhydroxyethyl cellulose) sodium citrate was added to thehydrogen peroxide solution. The mixture thus obtained was agitatedcontinuously for 5 hours at 90° C. until the hydrogen peroxide was spentand then dried.

The intital and final viscosities in each case and the pH values of 2 wt% solutions of the products are shown in the table.

Initial viscosity Final viscosity pH-values of 2 wt. % Number (mPa/s)(mPa/s) aqueous solutions 11 400 30 4.8 12 7,100 90 4.7

1. A process for the production of low-viscosity water-soluble celluloseethers by oxidative decomposition of higher-viscosity cellulose etherswith hydrogen peroxide, comprising: (a) forming, under conditions ofintensive mixing and at temperatures of 65-125° C., a mixturecomprising, (i) higher-visocity cellulose ethers, and (ii) an aqueoussolution of hydrogen peroxide which is present in an amount of 0.5 to1.8 wt % in relation to the dry cellulose ether, the solid content ofthe mixture is no lower than 25 wt % in relation to the total quantityof the mixture; and (b) agitating continuously the mixture of step (a)at temperatures of 65-125° C. until approximately at least 90% of thehydrogen peroxide has been spent, wherein during or after thedecomposition reaction, the pH value of the mixture of step (a) is setat more than 4.5, by adding to said mixture a second aqueous solutionwhich has a pH of 5 to 12, provided that when said second aqueoussolution is added during the decomposition reaction said second aqueoussolution may optionally contain, in solution, the hydrogen peroxiderequired for the decomposition reaction.
 2. The process of claim 1wherein said mixture of step (a) is formed by adding aqueous hydrogenperoxide in portions.
 3. The process of claim 1 wherein ahigher-viscosity cellulose ether having a dry cellulose ether content of35-80wt %, in relation to the total quantity of cellulose ether andsolvent, is used.
 4. Process for the production of low-viscositywater-soluble cellulose ethers according to any one of claims 1, 2, or3, characterised in that the water soluble cellulose ether iscarboxymethyl cellulose, hydrophobically modified carboxymethylcellulose, hydroxyethyl carboxymethyl cellulose, sulfoethyl cellulose,hydrophobically modified sulfoethyl cellulose, hydroxyethyl sulfoethylcellulose, hydrophobically modified hydroxyethyl sulfoethyl cellulose,hydroxyethyl cellulose, hydrophobically modified hydroxyethyl cellulose,methyl cellulose, methylhydroxyethyl cellulose, methylhydroxyethylsulfoethyl cellulose, hydrophobically modified methythydroxyethylcellulose, methylhydroxypropyl cellulose, hydroxypropyl cellulose ormixtures thereof.
 5. Process for the production of low-viscositywater-soluble cellulose ethers according to any one of claims 1, 2, 3 or4, in that the water-soluble cellulose ether is methylcellulose, methylhydroxyethyl cellulose, hydrophobically modified methyl hydroxyethylcellulose, methyl hydroxypropyl cellulose, hydroxypropyl cellulose ormixtures thereof and water-wet filter cakes of these cellulose ethers,as obtained after washing and separation, are used.
 6. Process for theproduction of low-viscosity water-soluble cellulose ethers according toany one of claims 1, 2, 3, or 4 characterised in that the water solublecellulose ether is methyl hydroxyethyl cellulose or methyl hydroxypropylcellulose and water-wet filter cakes of the cellulose ethers, asobtained after washing and separation, are used.
 7. The process of claim1 wherein a higher-viscosity cellulose ether having a dry celluloseether content of 40 to 55 wt. %, in relation to the total quantity ofcellulose ether and solvent, is used.
 8. The process of claim 1 whereinthe pH value of the mixture of step (a) is set at 6 to
 7. 9. The processof claim 1 wherein said second aqueous solution comprises a memberselected from the group consisting of sodium dihydrogen phosphate,sodium hydrogen phosphate, sodium phosphate, sodium carbonate, sodiumhydrogen carbonate, alkali salts of citric acid, alkali salts ofsuccinic acids and combinations thereof.